Most modem electronic devices manufactured today contain at least one electrical signal line which is an unwanted source of electrical “noise”, thereby adversely affecting other electronic circuits, both within and external to the electronic device. Generally speaking, this noise exists in the form of electromagnetic interference (EMI) of nearby electrical signals by the offending electrical signal. This EMI may be conducted from the offending electrical signal line to others by way of an electrically conductive path. Alternately, the interference may be radiated from the offending electrical signal line to nearby circuits without the benefit of a directly conductive connection. Oftentimes, the result of such radiated or conducted noise is erroneous or improper operation of the circuit being affected by the EMI, due primarily to unexpected voltage changes in the affected circuit. As a result, protecting electrical circuits from EMI that is generated by other signal lines has long been an important facet of the electronic circuit and device design process.
One example of a source of such noise is a digital clock signal, which is an internally generated oscillating signal that is commonly employed to drive and synchronize various circuits, such as microprocessors, peripheral controllers, and other logic integrated circuits (ICs) within an electronic device. Such clock signals typically take the form of a square wave operating at a specific frequency, thus often generating substantial noise at that fundamental frequency and at various harmonics of the fundamental frequency. These clock signals often are required at numerous locations within an electronic device, requiring extensive routing throughout the device, as well as multiple signal drivers due to the extensive number of circuits such a signal often must drive. Thus, the task of reducing the effects of such a widely distributed high-frequency signal to reduce the deleterious effects of the signal on surrounding circuitry is problematic at best. Furthermore, as electronic technology progresses, newer electronic devices tend to utilize clock signals with higher frequencies than those devices of previous technological generations, making the task of mitigating the effects of the generated noise even more difficult.
Several methods of protecting circuits from EMI generated by these oscillating signals have been employed previously. Many such methods involve protecting the sensitive circuits of the electronic device from the noisy signal source. For example, the electronic circuit designer often attempts to structure the physical layout of the electronic circuits on a printed circuit board (PCB) so that the generated EMI of the noisy signal will have an attenuated effect on other surrounding circuits. Such efforts include physically routing the offending signal remotely from other sensitive signal lines and circuits, utilizing additional ground planes within the PCB to electrically shield and separate the noisy signal from surrounding circuits, and the like. Unfortunately, such efforts normally require exorbitant amounts of a PCB designer's time and effort, and are also error-prone, requiring multiple circuit design revisions in order to reduce sufficiently the effects of the noise on the device.
Other similar solutions involve more substantive circuit additions to shield radiated and conducted noise from circuits that are sensitive to that noise. These additions include the use of large and complex filters on the PCB, chokes, additional metal shielding, shielded cables, and so on.
In contrast to the solutions above, more recent approaches to the problem involve changing the nature of the offending clock signal itself to make that signal less of a noise source to surrounding circuitry. For example, one proposed solution has been to “dither” the clock signal by adding a small noise signal to the clock signal itself. Dithering of the clock signal results in displacing the frequency spectrum of the offending noise a small amount, but does not lower the power level of the frequency spectrum. This solution has been utilized in devices in which other circuits within the device are sensitive to noise at particular frequencies, because the small displacement in the frequency spectrum of the clock signal may aid in reducing the effects of the clock noise on that circuit. However, many electronic devices are susceptible to noise across a wide range of frequencies, making this solution inapplicable in such cases. For example, dithering of the clock signal is particularly ineffective for electronic devices such as electronic test and measurement instruments, which often are employed to investigate electronic signals over a very wide band of the frequency spectrum.
Other prior art solutions, such as those indicated in “Current control technique for improving EMC in power converters,” ELECTRONIC LETTERS, Vol. 37, No. 5, pp. 274-275 (Mar. 1, 2001) by Giral et al., and “Improvement of power supply EMC by chaos,” ELECTRONIC LETTERS, Vol. 32, No 12, p. 1045 (Jun. 6, 1996) by Deane et al., focus on the use of chaotic control of direct-current to direct-current (DC-DC) power converters to reduce the electromagnetic interference normally generated by such circuits. Such solutions succeed in reducing the peaks of the frequency spectrum due to the switching signal associated with such converters by spreading out the power of the spectrum at the fundamental and harmonic frequencies. However, such solutions typically do not ensure failsafe operation of the circuit being driven by the offending signal due to its chaotic nature. For example, in the case of the DC-DC power converter, an inductor is employed as an energy storage element that is intermittently energized by way of a switch. The operation of the switch is controlled by way of a small control circuit that is normally driven by a periodic signal, conditioned by the voltage output of the converter. Adding chaotic control as described by the prior art does not guarantee that the switch will not remain in the closed position, thus potentially causing permanent damage to the inductor by way of sustained electrical current. Similar restrictions normally apply to other periodic signals, such as digital clock signals, which typically must operate within a few percent of a specified frequency.
Another solution, identified by Cahill in U.S. Pat. No. 5,263,055, entitled “APPARATUS AND METHOD FOR REDUCING HARMONIC INTERFERENCE GENERERATED BY A CLOCK SIGNAL”, implements a periodic clock signal that is frequency modulated, or alternately, phase modulated, by the output of a pseudorandom noise signal generator. While the power spectral energy of the fundamental and harmonic frequencies of the periodic clock signal is reduced, no control mechanism is present which ensures that the changing frequency of the modulated signal remains within the limits required of the circuit being driven by that signal.
From the foregoing, despite previous attempts to mitigate or reduce EMI generated by periodic electronic signals, a need still exists for a reliable method of reducing the EMI generated by oscillating signals exhibiting at least some degree of periodic behavior. Such a method should both reduce the EMI generated by the oscillating signal while ensuring that the signal retains the characteristics required for proper operation of the circuits it drives.